What constitutes GOOD Science Fiction?

This is a question I've been pondering for a while. Recently on some site I noticed a science fiction group, and the only requirement was the books had to be science fiction. Then they proceeded to give an odd definition. This isn't the first time I've heard the topic discussed, so I thought I'd throw in my two bits, but instead of defining science fiction, I thought I'd tackle good science fiction. After all, who wants to waste time with the bad.

I understand everyone has their own tastes and drethers, but for me, I like science fiction that is set in a world I can identify with. I'm a huge fan of the old Dimension X and X-Minus One radio programs. Most of those took the modern world (1950s and 60s modern that it) and just had one scifi element that created the story. I thought those were great! I could easily join that world and explore the "element x" along with the characters in the story.

Of course, you can set the story in a fantastic setting on a far off world and still be relatable. Take Star Trek. The really great episodes explore philosophical ideas and use the scifi elements as window dressing. Most of those stories could be set in other places at other times, and the plot and major themes would still be the same. (They set many of the episodes in other times and places like the old west or 1960's Earth).

I also like interesting characters that are well developed. That's probably why I'm not a big fan of strange aliens, because they are usually very two dimensional and underdeveloped. Being strange is not enough to generate interest. The aliens need to have real character.

I also like stories that pick and idea and explore different sides of it without getting preachy or overly didactical. Stories that are mere propaganda or try to beat their point in my head with a hammer are less interesting. The idea can be philosophical like Star Trek, or it can be scientific theory like a Charles Sheffield novel. Or it can be political like Episodes I, II, and III of Star Wars.